Historical Background … From World Wide Words: Quite how I’m your huckleberry came out of all that with the sense of the man for the job isn’t obvious. It seems that the word came to be given as a mark of affection or comradeship to one’s partner or sidekick. There is often an identification of oneself as a willing helper or assistant about it, as here in True to Himself, by Edward Stratemeyer, dated 1900: “ ‘I will pay you for whatever you do for me.’ ‘Then I’m your huckleberry. Who are you and what do you want to know?’ ”. Despite the obvious associations, it doesn’t seem to derive directly from Mark Twain’s books….

That ended it. Holliday went into the saloon. Ringo withdrew across the street.

According to Victoria ….

Huckleberries hold a place in archaic American English slang. The tiny size of the berries led to their use as a way of referring to something small, often affectionately as in the lyrics of Moon River. The phrase “a huckleberry over my persimmon” was used to mean “a bit beyond my abilities”. “I’m your huckleberry” is a way of saying that one is just the right person for a given job. The range of slang meanings of huckleberry in the 19th century was fairly large, also referring to significant persons or nice persons.